1. Field of the Invention
The present invention relates to the process of fabricating structures having severe topologies including, but not limited to, semiconductor structures. In particular, the present invention relates to an arrangement to planarize such structures to enable effective photolithography.
2. Description of the Prior Art
A variety of structures, including semiconductor structures, are fabricated using a series of steps to build layers of materials, each layer having properties determined by the fabrication process. Among many steps, the process includes the addition of material to the surface of a semiconductor wafer substrate and the removal of certain portions of that material to define various active (conductive) and insulative regions in subsequent steps. The relationship among those various regions defines the characteristics of the structure. Material is added and removed, molecules inserted in certain sections, and thermal processing is carried out before conformal materials are applied to create a relatively flat surface of the structure substantially enclosing the various regions of material. The basic processing steps involved are generally well known to those skilled in the art and vary as a function of the particular devices desired.
In order to create devices that function as desired, it is important to establish the locations and sizes of the various regions as accurately as possible. The siting of those three-dimensional regions is defined either by self-alignment—the positioning of adjacent and/or covering layers of material—or by photolithography. Photolithography is the process of transferring an image to the surface of the semiconductor substrate by means of a light sensitive polymeric film. The film is first applied to the surface of a substrate and a mask is then used to establish opaque and transparent regions matching the desired pattern to be formed in the substrate. The mask is fabricated such that when light passes through the transparent regions, the underlying film is either cured or made soluble in those areas exposed to the light source. The film is then subjected to a chemical solution to remove unexposed film or exposed film, dependent upon the particular film type (positive or negative) employed. With that patterned film in place, process steps may be carried out at those sites where the film has been removed. The regions of the substrate where the film remains are “protected” from the process operations. For example, semiconductor material in the unprotected area may be removed by etching.
Photolithography works best when the film is applied to a flat surface. When the film is applied to a flat surface, it settles to a thickness that is substantially uniform. The light required to cure or make the film more soluble is generated by a source that produces enough energy to change the condition of the film completely through its thickness. Existing photolithographic equipment or “steppers” produce light of a wavelength suitable to change the chemical state of the film. The light source of the stepper is arranged so that the focal point of the generated light beam is preferably targeted at the center of the thickness of the film. If the focal point is established too near the surface of the film, the film may be underexposed near the surface of the substrate. If the focal point is established near the surface of the substrate, the film may be badly exposed near the film's surface. The particular focal point to be established is dependent upon the wavelength of the light from the source, the thickness of the film, and any non-planarity of the substrate to which the film is applied.
When the film thickness is uniform and substantially on a single plane, and the source wavelength is known, required “minimum energy to clear” is constant. In addition, when the substrate surface is substantially on one plane, the required Depth of Focus (DOF) is also constant. However, when the substrate surface is not completely flat; that is, when it is topographical, the film layer is not of uniform thickness and the height of the film surface varies. Non-uniformity of thickness may occur when there are pockets, channels, slots, etc., in the surface of the substrate. In those cases, the film material, when first applied to the substrate, fills in such areas and will necessarily be thicker in those localized areas. The thickest part of the film determines the greatest amount of light energy required to convert the film to the required properties either to make it sufficiently soluble or sufficiently cured. That required energy is the minimum energy to clear. As might be expected, the minimum energy to clear is greatest where the film is thickest and least where it is thinnest. However, steppers of the type used in most commercial semiconductor fabrication applications, for example, do not provide selectable localized changes in the light energy applied to the film. Attempts to supply enough energy to adequately affect the deepest part of the film will overexpose the thinnest part, rendering the film unacceptable.
When the substrate surface is non-planar, the film may be of substantially uniform thickness on the entire surface except, perhaps, where it transitions from one plane to another. However, when the overall height of the substrate varies from one region to another, the light source's distance from the substrate surface is variable. Since commercially available steppers direct the light from a fixed position, the focus setting or DOF established to affect the film at one region of the substrate is not suitable to affect the film at a region of different height. A solution to this problem involves creating multiple focus settings as a function of the number of surface height changes. Two or more superimposed lithographic exposures are used to create a deeper effective DOF. This is suitable for surface height variations of modest difference. However, for greater changes, image contrast is compromised such that the film is unsuitable for use in some regions.
There has been recent interest in possible commercial applications of the combination of microelectronics and micromechanical structures. These miniaturized structures, sometimes referred to as Micro-Electro-Mechanical Systems (MEMS), may be employed as optical controllers, such as for miniaturized mirrors; as pressure sensors, such as for automotive applications; as pumps, motors, chemical sensor controllers, controllable infusion devices for medical applications, among an array of uses requiring control systems and mechanized elements scaled to the size of integrated circuitry. Although such types of devices have been developed on a very small scale for certain specific regulated applications, broader applicability requires suitable fabrication processes to make them commercially viable. The nature of these microelectronic/micromechanical systems, however, necessarily involves the formation of structures having severe topologies. That is, the microscopic-scaled structures required to create the mirrors, or pumps, or motors, etc., as well as the related control electronics, will require the creation of deep wells, high pedestals, and the like, resulting in severe differences in photolithographic film thickness across the entire structure. Such severe differences cannot always be resolved with existing commercial steppers and semiconductor fabrication processes. That is, the dimensions of the severe structural variations may in some cases be too great to enable resolution of DOF requirements.
Therefore, what is needed is a system and related process steps to improve or enable microlithography on severe, non-flat (non-uniform planar) topologies using commercial fabrication equipment. What is needed in particular is a system and related process steps to minimize variations in minimum energy to clear and required DOF for photoresist film applied to semiconductor-based structures having severe topologies. Further, what is needed is such a system and related steps that may be incorporated into conventional structure fabrication methods, including those employed in making semiconductor structures.